The present invention relates generally to semiconductor laser diodes. In particular, the present invention is a laser diode fabricated from Group II-VI compound semiconductors which emits coherent radiation in the blue and green portion of the spectrum.
Semiconductor laser diodes are generally known and disclosed, for example, in Chapter 12 of Sze, Physics of Semiconductor Devices, 2nd ed. pp. 681-742 (1981). To date, most commercially available laser diodes are fabricated from Group III-V compound semiconductors and their alloys such as GaAs and AlGaAs. These devices emit light in the infrared and red portions of the spectrum, e.g., at wavelengths between 630 and 1550 nm. Laser diodes of these types are used in a wide range of applications such as communications, recording, sensing and imaging systems.
Nonetheless, there are many applications for which the wavelength of light generated by infrared and red laser diodes is not suitable. Commercially viable laser diodes which emit radiation at shorter wavelengths, for example in the green and blue portions of the spectrum (i.e., at wavelengths between 590 and 430 nm) would have widespread application. Shorter wavelength laser diodes would also increase the performance and capabilities of many systems which currently use infrared and red laser diodes.
Wide band gap II-VI semiconductors and alloys, and in particular ZnSe, have for many years been called promising materials for the fabrication of blue and green light emitting devices. In the 1960's, laser action was demonstrated in several II-VI semiconductors using electron-beam pumping techniques. Colak et al., Electron Beam Pumped II-VI Lasers, J. Crystal Growth 72, 504 (1985) includes a review of this work. There have also been more recent demonstrations of photopumped and electron-beam pumped lasing action from epitaxial II-VI semiconductor materials. See e.g., Potts et al., Electron Beam Pumped Lasing In ZnSe Grown By Molecular-Beam Epitaxy, Appl. Phys. Lett., 50, 7 (1987) and Ding et al., Laser Action In The Blue-Green From Optically Pumped (Zn,Cd)Se/ZnSe Single Quantum Well Structures, Appl. Phys. Lett. 57, p. 2756 (1990). As research on wide band gap II-VI semiconductor devices progressed, several key technological difficulties were identified. These difficulties included: 1) the inability to produce low-resistivity p-type ZnSe and related alloys; 2) the inability to form device-quality ohmic contacts to p-type ZnSe and related alloys, and 3) the lack of a suitable lattice-matched heterostructure material system.
Modern epitaxial growth techniques such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) are now used to fabricate device quality undoped and n-type ZnSe layers, typically on GaAs substrates. The growth of low resistivity p-type ZnSe using Li and N (NH:) as dopants has also been reported. For some time it appeared that the upper limit of obtainable net acceptor concentrations (N.sub.A -N.sub.D) was about 10.sup.17 cm.sup.-3. Recently, however, significantly greater net acceptor concentrations have been achieved in ZnSe:N grown by MBE using nitrogen free radicals produced by an rf plasma source. See e.g., Park et al., P-type ZnSe By Nitrogen Atom Beam Doping During Molecular Beam Epitaxial Growth, Appl. Phys. Lett. 57, 2127 (1990) and copending Park et al. U.S. patent application Ser. No. 07/573,428 filed Aug. 24, 1990 and entitled Doping Of IIB-VIA Semiconductors During Molecular Beam Epitaxy. The largest net acceptor concentration in ZnSe achieved through the use of these techniques is 2.times.10.sup.18 cm.sup.-3. Using these technologies, rudimentary blue light emitting diodes have been reported by several laboratories. See e.g., the Park et al. Appl. Phys. Lett. article referred to immediately above.
Of the wide band gap II-VI semiconductor systems that are reasonably well developed, i.e., ZnSeTe, CdZnSe, ZnSSe and CdZnS, only CdZnS-ZnSe offers a lattice-matched system. Unfortunately, this system offers only a very small band gap difference (about 0.05 eV), which is far too small for the carrier confinement needed for simple double heterostructure laser diodes. Therefore, to achieve a band gap difference greater than 0.2 eV, it would be necessary to use a strained-layer system (e.g., ZnSe-Cd.sub.x Zn.sub.1-x Se with x.gtoreq.0.2). To prevent misfit dislocations which degrade the luminescence efficiency, the thickness of the strained layer should be kept less than the critical thickness. However, a simple double heterostructure laser made accordingly would have an active layer thickness so thin (due to the large mismatch required for sufficient band gap difference) that the optical mode would be very poorly confined. Thus, the confinement factor (overlap between the optical mode and the light generating region) would be small, and substrate losses would be high, causing prohibitively high threshold currents. Therefore, simple double heterostructure laser diodes are not practical in these wide band gap II-VI materials.
For these reasons, there have been no known demonstrations of laser diodes fabricated from II-VI compound semiconductors. Commercially viable laser diodes of this type would be extremely desirable and have widespread application.